1. Field of the Invention
This invention relates to a process for producing cathode aluminum capacitor foil, and more particularly to an improved process for post etch treatment of cathode aluminum foil etched in an acid and/or salt electrolyte solution under action of an AC and/or DC current and thereafter treating the etched foil to substantially completely remove chloride ions, increase the capacitance of the foil, and reduce the capacitance decay rate of the foil.
2. Description of the Prior Art
As is known, electrolytic capacitors are conventionally constructed by winding an anode and a cathode foil into a coil, with adjacent surfaces of the two foils being separated by spacers such as sheets of paper material. The wound coil is inserted into an aluminum container which is filled with a liquid electrolyte solution and sealed. Connector tabs joined to the respective foils project outwardly through the sealed container to enable connecting the capacitor in a circuit.
Both the anode and cathode foils are typically electrolytically etched to increase the surface area, and thereby increase the capacitance of the foil. The anode foil is anodized to form an oxide coating over its entire surface. The cathode foil is not anodized although its surface may be subject to limited oxidation during storage following the etching process and prior to the foil being installed in a capacitor.
Increased emphasis is being placed upon obtaining maximum capacitance of the capacitor foil and thereby enable reduction in size of the capacitor. While size reduction inherently produces a saving in material and consequently in cost, such size reduction may be of primary concern particularly where the capacitor is to be used in miniature circuits of the type presently in common use. Although many improvements have been made in the electrolytic etching art which have resulted in increased foil capacitance, there remains a need for further improvements. Also, since it is not always possible to immediately incorporate etched foil into a sealed capacitor, it is important that deterioration of the capacitance during storage is maintained at a minimum. This is particularly true for cathode foil which is more adversely affected by the buildup of a very thin oxide film during storage than anode film which is intentionally anodized.
Usability of capacitor foil which has been etched in a chloride electrolyte solution is enhanced by efficiently removing the chlorides from the foil surface. This cleaning process conventionally comprises thoroughly washing or rinsing the foil promptly after it exits the electrolyte solution. It is also known to immerse the etched, washed foil in an aqueous solution of nitric acid for a relatively short time to remove chlorides from the etched pores. The foil must not be subjected to the action of the nitric acid for more than a very short time or a reduction in capacitance may result due to the nonelectrolyte etching, or polishing effect of the acid; however, the capacitance is actually increased during initial exposure due, it is believed, to the acid removing chlorides and oxides from the very fine pores or channels produced during electrolytic etching and thereby increasing the effective surface area. The more efficient removal of chlorides in the nitric acid solution, followed by effective water washing of the acid from the foil, will also slightly increase the storage life of the etched foil.
It is also known, for example from U.S. Pat. No. Re. 28,015, to further treat foil which has been water rinsed following etching by immersing the foil in a boiling aqueous solution of CrO.sub.3 and H.sub.3 PO.sub.4 to strip aluminum oxide from the foil surface, then passifying the stripped, etched foil by immersing it in a boiling aqueous solution of CrO.sub.3, H.sub.2 PO.sub.4 --, and SiO.sub.3 .tbd.. This passivation treatment is intended to retard the formation of hydroxide on the aluminum surface in the presence of water and to increase the storage life of the anode foil prior to the anodizing step preparatory to constructing the electrolytic capacitor. Further, the passivation step is said to retard corrosion in the finished capacitor.